Spontaneous periodic pulsation of contact line in oil/water system--frequency control with divalent cations and applied voltage

Periodic oscillatory change of hydrophilicity (or hydrophobicity) of a glass surface was studied. A glass capillary was immersed normally at an oil/water interface. The water phase contained the cationic surfactant trimethyloctadecylammoniumchloride, and the oil phase contained bis(2ethylhexyl) phosphate. Adsorption of the surfactant molecules and their desorption via anionic chemicals dissolved in the oil generated a gradual wetting by the water, followed by a rapid wetting by oil. The three phase contact line exhibited a pulse-like motion that continued, at least for a few minutes. The frequency depended on the cation species dissolved in water and the applied voltage across the oil/water interface. Four kinds of cations, Mg(2+), Ca(2+), Sr(2+) and Ba(2+) were used. While the frequency order was Ba(2+)>Sr(2+)>Mg(2+), the Ca(2+)-containing interface did not show any motion irrespective of the applied voltage. There was a threshold voltage and concentration of anionic chemical that was necessary for the onset of this motion. The pulsation mechanism and its ion selectivity are also discussed. This interfacial motion was a typical nonlinear oscillation with an ion-selective nature. In this regard, this interfacial motion had biomimetic characteristics.     

Surfactant behavior of "ellipsoidal" dicarbollide anions: a molecular dynamics study

We report a molecular dynamics study of cobalt bis(dicarbollide) anions [(B(9)C(2)H(8)X(3))(2)Co](-) (XCD(-)) commonly used in liquid-liquid extraction (X = H, Me, Cl, or Br), showing that these anions, although lacking the amphiphilic topology, behave as anionic surfactants. In pure water, they display "hydrophobic attractions", leading to the formation of aggregates of different sizes and shapes depending on the counterions. When simulated at a water/"oil" interface, the different anions (HCD(-), MeCD(-), CCD(-), and BrCD(-)) are found to be surface active. As a result, the simulated M(n+) counterions (M(n+) = Na(+), K(+), Cs(+), H(3)O(+), UO(2)(2+), Eu(3+)) concentrate on the aqueous side of the interface, forming a "double layer" whose characteristics are modulated by the hydrophobic character of the anion and by M(n+). The highly hydrophilic Eu(3+) or UO(2)(2+) cations that are generally "repelled" by aqueous interfaces are attracted by dicarbollides near the interface, which is crucial as far as the mechanism of assisted cation extraction to the oil phase is concerned. These cations interact with interfacial XCD(-) in their fully hydrated Eu(H(2)O)(9)(3+) and UO(2)(H(2)O)(5)(2+) forms, whereas the less hydrophilic monocharged cations display intimate contacts via their X substituents. The results obtained with the TIP3P and OPLS models for the solvents are confirmed with other water models (TIP5P or a polarizable 4P-Pol water) and with more polar "oil" models. The importance of interfacial phenomena is further demonstrated by simulations with a high oil-water ratio, leading to the formation of a micelle covered with CCD's. We suggest that the interfacial activity of dicarbollides and related hydrophobic anions is an important feature of synergism in liquid-liquid extraction of hard cations (e.g., for nuclear waste partitioning).     

Complexes of bivalent metal cations in electrospray mass spectra of common organic compounds

In the standard electrospray ionization mass spectra of many common, low molecular mass organic compounds dissolved in methanol, peaks corresponding to ions with formula [3M + Met](2+) (M = organic molecule, Met = bivalent metal cation) are observed, sometimes with significant abundances. The most common are ions containing Mg(2+), Ca(2+) and Fe(2+). Their presence can be easily rationalized on the basis of typical organic reaction work-up procedures. The formation of [3M + Met](2+) ions has been studied using N-FMOC-proline methyl ester as a model organic ligand and Mg(2+), Ca(2+), Sr(2+), Ba(2+), Fe(2+), Ni(2+), Mn(2+), Co(2+) and Zn(2+) chlorides or acetates as the sources of bivalent cation. It was found that all ions studied form [3M + Met](2+) complexes with N-FMOC-proline methyl ester, some of them at very low concentrations. Transition metal cations generally show higher complexation activity in comparison with alkaline earth metal cations. They are also more specific in the formation of [3M + Met](2+) complexes. In the case of alkaline earth metal cations [2M + Met](2+) and [4M + Met](2+) complex ions are also observed. It has been found that [3M + Met](2+) complex ions undergo specific fragmentation at relatively low energy, yielding fluorenylmethyl cation as a major product. [M + Na](+) ions are much more stable and their fragmentation is not as specific.     

Effect of Na+, Mg2+, and Zn2+ chlorides on the structural and thermodynamic properties of water/n-heptane interfaces

The effect on the structural and thermodynamic properties in water/n-heptane interfaces on addition of NaCl, MgCl(2), and ZnCl(2) has been examined through five independent 100-ns molecular dynamics simulations. Results indicate that the interfacial thickness within the framework of the capillary-wave model decreases on addition of electrolytes in the order Na(+) < Mg(2+) < Zn(2+), whereas the interfacial tension increases in the same order. Ionic density profiles and self-diffusion coefficients are strongly influenced by the strength of the first hydration shell, which varies in the order Na(+) < Mg(2+) < Zn(2+). On the other hand, the Cl(-) behavior, that is, diffusion and solvation sphere, is influenced by its counterion. Accordingly, cations are strongly expelled from the interface, which is especially remarkable for the small divalent cations. This fact alters the water geometry near the interface and in a lesser extent n-heptane order and number of hydrogen bonds per water molecule close to the interface.     

Some aspects of stability of multiple emulsions in personal cleansing systems

The two dominant factors that were found to affect the stability of multiple emulsions in high HLB surfactant systems are the osmotic pressure imbalance between the internal aqueous phase and the external aqueous phase, and the adsorption/desorption characteristics of the emulsifier/surfactant film at the oil/water interface. Synergistic interaction between the low HLB emulsifier and the high HLB surfactant that produces very low interfacial tension of the order of 10(-2) mN/m at the oil/water interface was found to occur in some of the systems investigated. Long term stability was observed in multiple emulsion containing these systems. However, no synergy was observed in systems in which either the oil or the emulsifier, or both, contained unsaturated chains. In fact, desorption of the adsorbed surfactant film was observed in systems containing unsaturated chains. The observed desorption from the interface of the emulsifier in these systems was attributed mainly to the inability of the unsaturated chains to form a close packed, condensed interfacial film. Presence of closely packed, condensed interfacial film is necessary to prevent solubilization of the adsorbed low HLB emulsifier by the high HLB surfactant. Multiple emulsions prepared using systems containing unsaturated hydrocarbons were highly unstable.     

Relationship between interfacial transfer and adsorption-desorption of surface-active bis-ammonium ions at a liquid/liquid interface

Cyclic voltammograms and interfacial tension-applied potential curves were recorded at the interface between water containing surface-active bis-quaternary ammonium ions, bis-A(2+), and an organic solvent such as 1,2-dichloroethane or nitrobenzene. An ordinary diffusion-controlled voltammetric wave for the transfer of bis-A(2+) from aqueous phase to organic phase, the first wave, was followed by a typical adsorption-related wave, the second wave. It was found from the potential dependence of the interfacial tension of bis-A(2+) that the second wave was due to the desorption of bis-A(2+) toward the organic phase. The influence of the structure of bis-A(2+) on voltammograms was investigated, and the potential for the first wave was found to depend on both the length of the side chain and that of the spacer chain, whereas the potential for the second wave depended on the latter only. The thermodynamic relations among three processes of the ion transfer, adsorption, and desorption were discussed based on the experimental results.     

Marangoni flow of Ag nanoparticles from the fluid-fluid interface

Fluid flow is observed when a volume of passivated Ag nanoparticles suspended in chloroform is mixed with a water/ethanol (v/v) mixture containing acidified 11-mercaptoundecanoic acid. Following mechanical agitation, Ag nanoparticles embedded in a film are driven from the organic-aqueous interface. A reddish-brown colored film, verified by transmission electron microscopy to contain uniformly dispersed Ag nanoparticles, is observed to spontaneously climb the interior surface of an ordinary, laboratory glass vial. This phenomenon is recorded by a digital video recorder, and a measurement of the distance traveled by the film front versus time is extracted. Surface (interfacial) tension gradients due to surfactant concentration, temperature, and electrostatic potential across immiscible fluids are known to drive interface motion; this well-known phenomenon is termed Marangoni flow or the Marangoni effect. Experimental results are presented that show the observed mass transfer is dependent on an acid surfactant concentration and on the volume fraction of water in the aqueous phase, consistent with fluid flow induced by interfacial tension gradients. In addition, an effective desorption rate constant for the Marangoni flow is measured in the range of approximately 0.01 to approximately 1 s(-1) from a fit to the relative film front distance traveled versus time data. The fit is based on a time-dependent expression for the surface (interface) excess for desorption kinetics. Such flow suggests that purposeful creation of interfacial tension gradients may aid in the transfer of 2- and 3-dimensional assemblies, made with nanostructures at the liquid-liquid interface, to solid surfaces.     

Ion exchange of Pb(2+), Cu(2+), Fe(3+), and Cr(3+) on natural clinoptilolite: selectivity determination and influence of acidity on metal uptake

In the present study ion exchange of Pb(2+), Cu(2+), Fe(3+), and Cr(3+) on natural Greek clinoptilolite was examined in terms of selectivity toward the above heavy metals in single- and multicomponent solutions in batch systems. Also examined are the influence of clinoptilolite on solution acidity and the effect of acidity on the ion exchange process. Clinoptilolite increases solution acidity due to the exchange of H(+) cations with the cations initially present in its structure. H(+) cations should be considered as competitive ones in ion exchange processes, and consequently ion exchange of metals is favored at high acidity values. Cu(2+) and Cr(3+) are the most sensitive cations with respect to acidity. Selectivity determination demonstrates that the selectivity at total concentration 0.01 N and acidity 2 in both single- and multicomponent solutions is following the order Pb(2+)>Fe(3+)>Cr(3+) > or =Cu(2+). This order is set since the first days of equilibration. However, Cu(2+) shows remarkable changes in selectivity and generally its uptake and selectivity are increasing with time. On the other hand selectivity in single metal solutions where acidity is not adjusted is following the order Pb(2+)>Cr(3+)>Fe(3+) congruent with Cu(2+).     

Interactions between synthetic and indigenous naphthenic acids and divalent cations across oil–water interfaces: effects of addition of oil-soluble non-ionic surfactants

Interactions between naphthenic acids and divalent metal cations across model oil–alkaline water interfaces were investigated by correlating changes in dynamic interfacial tension (IFT), to plausible reaction mechanisms. The measurements were carried out by using a CAM 200 optical instrument, which is based on the pendant drop technique. The naphthenic acids used were synthesised model compounds as well as commercial acid mixtures from crude distillation and extracted acid fractions from a North Sea crude oil. The divalent cations involved Ca²⁺, Mg²⁺, Sr²⁺, and Ba²⁺, which are all common in co-produced formation water and naphthenate deposits. The results show that the dynamic IFT strongly depends on naphthenic acid structure, type of divalent cation, and the concentration of the compounds as well as the pH of the aqueous phase. Introducing divalent cations to systems involving saturated naphthenic acids caused mostly a permanent lowering of the IFT. The decline in IFT is due to electrostatic attraction forces across the interface between the cations in the aqueous phase and the carboxylic-groups at the o/w interface, which cause a higher interfacial density of naphthenic acid monomers. The permanent lowering in IFT is likely due to formation of positively charged monoacid complexes, which possess high interfacial activity. On the other hand, in the case of the aromatic model compounds, the cations affected the IFT differently. This is mainly discussed in light of degree of cation hydration and steric conditions. Various oil-soluble non-ionic surfactant mixtures were also introduced to systems involving a model naphthenic acid and Ca²⁺ in order to investigate how the interfacial competition affected the local interactions. Based on the behaviour of dynamic IFT, probable inhibition mechanisms are discussed.Electronic Supplementary Material Supplementary material is available for this article at     

Chemical oscillation with periodic adsorption and desorption of surfactant ions at a water/nitrobenzene interface.

Chemical oscillations with periodic adsorption and desorption of surfactant ions, alkyl sulfate ions, at a water/nitrobenzene interface have been investigated. The interfacial tension was measured with a quasi elastic laser scattering (QELS) method and the interfacial electrical potential was obtained. We found that this oscillation consists of a series of abrupt adsorptions of ions, followed by a gradual desorption. In addition, we observed that each abrupt adsorption was always accompanied by a small waving motion of the liquid interface. From the analysis of the video images of the liquid interface or bulk phase, we could conclude that each abrupt adsorption is caused by nonlinear amplification of mass transfer of ions from the bulk phase to the liquid interface by a Marangoni convection, which was generated due to local adsorption of the surfactant ions at the liquid interface that resulted in the heterogeneity of the interfacial tension. In the present paper, we describe the mechanism of the chemical oscillation in terms of the hydrodynamic effect on the ion adsorption processes, and we also show the interfacial chemical reaction with ion exchange during the ion desorption process.     

Mass transfer model of triethylamine across the n-decane/water interface derived from dynamic interfacial tension experiments

This publication presents a detailed experimental and theoretical study of mass transfer of triethylamine (TEA) across the n-decane/water interface. In preliminary investigations, the partition of TEA between n-decane and water is determined. Based on the experimental finding that the dissociation of TEA takes place in the aqueous and in the organic phase, we assume that the interfacial mass transfer is mainly affected by adsorption and desorption of ionized TEA molecules at the liquid/liquid interface. Due to the amphiphilic structure of the dissociated TEA molecules, a dynamic interfacial tension measurement technique can be used to experimentally determine the interfacial mass transport. A model-based approach, which accounts for diffusive mass transport in the finite liquid bulk phases and for adsorption and desorption of ionized TEA molecules at the interface, is employed to analyze the experimental data. In the equilibrium state, the interfacial tension of dissociated TEA at the n-decane/water interface can be adequately described by the Langmuir isotherm. The comparison between the theoretical and the experimental dynamic interfacial tension data reveals that an additional activation energy barrier for adsorption and desorption at the interface has to be regarded to accurately describe the mass transport of TEA from the n-decane phase into the aqueous phase. Corresponding adsorption rate constants can be obtained by fitting the theoretical predictions to the experimental data. Interfacial tension measurements of mass transfer from the aqueous into the organic phase are characterized by interfacial instabilities caused by Marangoni convection, which result in an enhancement of the transfer rate across the interface.     

New masking procedure for selective complexometric determination of copper(II)

A study has been made of a new masking procedure for highly selective complexometric determination of copper(II), based on decomposition of the copper-EDTA complex at pH 5-6. Among the various combinations of masking agents tried, ternary masking mixtures comprising a main complexing agent (thiourea), a reducing agent (ascorbic acid) and an auxiliary complexing agent (thiosemicarbazide or a small amount of 1,10-phenanthroline or 2,2'-dipyridyl) have been found most suitable. An excess of EDTA is added and the surplus EDTA is back-titrated with lead (or zinc) nitrate with Xylenol Orange as indicator (pH 5-6). A masking mixture is then added to decompose the copper-EDTA complex and the liberated EDTA is again back-titrated with lead (or zinc) nitrate. The following cations do not interfere: Ag(+), Hg(2+), Pb(2+), Ni(2+), Bi(3+), As(3+), Al(3+), Sb(3+), Sn(4+), Cd(2+), Co(2+), Cr(3+) and moderate amounts of Fe(3+) and Mn(2+). The notable feature is that consecutive determination of Hg(2+) and Cu(2+) can be conveniently carried out in the presence of other cations.     

Colorimetric test kit for Cu2+ detection

A coumarin-based colorimetric chemosensor 1 was designed and synthesized. It exhibits good sensitivity and selectivity for the copper cation over other cations such as Zn(2+), Cd(2+), Pb(2+), Co(2+), Fe(2+), Ni(2+), Ag(+), and alkali and alkaline earth metal cations both in aqueous solution and on paper-made test kits. The change in color is very easily observed by the naked eye in the presence of Cu(2+) cation, whereas other metal cations do not induce such a change. The quantitative detection of Cu(2+) was preliminarily examined.     

An iron(III) selective dendrite chelator based on polyamidoamine dendrimer modified with 4-bromo-1,8-naphthalimide

A new 4-bromo-1,8-naphthalimide-labelled polyamidoamine (PAMAM) dendrimer from zero generation has been synthesised and characterised. Its functional characteristics, determined in acetonitrile solvent are discussed. The ability of the dendrimer to detect metal cations has been evaluated in acetonitrile by monitoring the quenching of the fluorescence intensity. Different metal cations have been tested Co(2+), Ni(2+), Cu(2+) and Fe(3+) for the purpose. The results have shown clearly that only Fe(3+) could be efficiently detected using the dendrimer.     

Hydrodynamically induced chemical oscillation at a water/nitrobenzene interface

By measuring a time course of interfacial tension and interfacial electrical potential, we successfully observed oscillatory phenomena that were based on alternatively appearing adsorption and desorption processes of anionic surfactant molecules (sodium dodecyl sulfate (SDS)) at the water/nitrobenzene interface. These oscillation patterns were drastically modified by slightly changing the rate of SDS aqueous solution injection into the water phase. When 10 mM of SDS aqueous solution was injected at a low rate, for example, at less than 1 microl/min, abrupt adsorption was repeatedly followed by slow desorption of DS- ions; in other words, the sequence of the oscillation and relaxation processes was repeated. However, when it was injected at a higher rate, no remarkable periodic phenomenon occurred after the first oscillation. In addition, the rapid adsorption process was observed to be accompanied by a flip motion of the liquid/liquid interface and a flow along the interface. This is caused by a Marangoni convection that is brought about by the generation of heterogeneity of interfacial tension. Furthermore, by estimating the flow speed, it was determined that the faster flow tends to quench the periodic oscillation patterns.     

Incorporation of triazole into a quinoline-rhodamine conjugate imparts iron(iii) selective complexation permitting detection at nanomolar levels

Two new rhodamine based probes 1 and 2 for the detection of Fe(3+) were synthesized and their selectivity towards Fe(3+) ions in the presence of other competitive metal ions tested. The probe 1 formed a coloured complex with Fe(3+) as well as Cu(2+) ions and revealed the lack of adequate number of coordination sites for selective complexation with Fe(3+). Incorporation of a triazole unit to the chelating moiety of 1 resulted in the probe 2, that displayed Fe(3+) selective complex formation even in the presence of other competitive metal ions like Li(+), Na(+), K(+), Cu(2+), Mg(2+), Ca(2+), Sr(2+), Cr(3+), Mn(2+), Fe(2+), Co(2+), Ni(2+), Zn(2+), Cd(2+), Hg(2+) and Pb(2+). The observed limit of detection of Fe(3+) ions (5 × 10(-8) M) confirmed the very high sensitivity of 2. The excellent stability of 2 in physiological pH conditions, non-interference of amino acids, blood serum and bovine serum albumin (BSA) in the detection process, and the remarkable selectivity for Fe(3+) ions permitted the use of 2 in the imaging of live fibroblast cells treated with Fe(3+) ions.     

Properties of a mixed-valence (Fe(II))2(Fe(III))2 square cell for utilization in the quantum cellular automata paradigm for molecular electronics

The di-mixed-valence complex [{(eta(5)-C5H5)Fe(eta(5)-C5H4)}4(eta(4)-C4)Co(eta(5)-C5H5)]2+, 1(2+), has been evaluated as a molecular four-dot cell for the quantum cellular automata paradigm for electronic devices. The cations 1(1+) and 1(2+) are prepared in good yield by selective chemical oxidation of 1(0) and are isolated as pure crystalline materials. The solid-state structures of 1(0) and 1(1+) and the midrange- and near-IR spectra of 1(0), 1(1+), 1(2+), and 1(3+) have been determined. Further, the variable-temperature EPR spectra of 1(1+) and 1(2+), magnetic susceptibility of 1(1+) and 1(2+), M?ssbauer spectra of 1(0), 1(1+), and 1(2+), NMR spectra of 1(0), and paramagnetic NMR spectra of 1(1+) and 1(2+) have been measured. The X-ray structure determination reveals four ferrocene "dots" arranged in a square by C-C bonds to the corners of a cyclobutadiene linker. The four ferrocene units project from alternating sides of the cyclobutadiene ring and are twisted to minimize steric interactions both with the Co(eta(5)-C5H5) fragment and with each other. In the solid state 1(2+) is a valence-trapped Robin and Day class II compound on the 10(-12) s infrared time scale, the fastest technique used herein, and unambiguous evidence for two Fe(II) and two Fe(III) sites is observed in both the infrared and M?ssbauer spectra. Both EPR and magnetic susceptibility measurements show no measurable spin-spin interaction in the solid state. In solution, the NMR spectra show that free rotation around the C-C bonds connecting the ferrocene units to the cyclobutadiene ring becomes increasingly hindered with decreasing temperature, leading to spectra at the lowest temperature that are consistent with the solid-state structure. Localization of the charges in the cations, which is observed in the paramagnetic NMR spectra as a function of temperature, correlates with the fluxional behavior. Hence, the alignment between the pi systems of the central linker and the ferrocene moieties most likely controls the rate of electron exchange between the dots.     

Tuning surface tension and aggregate shape via a novel redox active fluorocarbon-hydrocarbon hybrid surfactant

This paper reports the surface and bulk properties of a newly designed redox active hybrid surfactant Fc(CH2)11N+(C2H5)2(CH2)2(CF2)5CF3 I- or FcFHUB, where Fc is ferrocene. This new surfactant displays strong surface tension lowering ability (31 mN/m) and low critical micelle concentration (0.03 mM in 100 mM Li2SO4). The minimum area per surfactant molecule at the interface is determined as 121 angstroms2/molecule. The electrochemical oxidation of ferrocene (Fc) to ferrocenium cationic (Fc+) leads to reversible changes in the surface and bulk properties of this surfactant. Following the oxidation, desorption of surfactant molecules from the surface of the solution takes place. This desorption of surfactant molecules gives rise to the oxidation-induced surface tension change up to 15 mN/m. Although this new molecule shows salt-insensitive behavior in its reduced form, the oxidized form of the surfactant shows slight sensitivity to the electrolyte concentration. The molecular structure of FcFHUB allows the formation of large aggregates in the form of coils at a temperature of 33 degrees C. When the temperature rises to 50 degrees C, the aggregates are determined to be in the vesicle structure. The oxidation of Fc to Fc+ disrupts large aggregates to the smaller aggregates at low temperatures. The oxidation of surfactant molecules at high temperature leads to disruption of the aggregates to monomers.     

New fluorogenic sensors for Hg2+ ions: through-bond energy transfer from pentaquinone to rhodamine

New pentaquinone derivatives 5 and 8 having rhodamine moieties have been designed and synthesized that undergo through-bond energy transfer (TBET) in the presence of Hg(2+) ions among the various cations (Cu(2+), Pb(2+), Fe(2+), Fe(3+), Zn(2+), Ni(2+), Cd(2+), Co(2+), Ag(+), Ba(2+), Mg(2+), K(+), Na(+), and Li(+)) tested in mixed aqueous media.     

The array and interfacial activity of sodium dodecyl benzene sulfonate and sodium oleate at the oil/water interface

There is a close correlation between the interfacial activity and the adsorption of the surfactant at the interface, but the detailed molecular standard information was scarce. The interfacial activity of two traditional anionic surfactants sodium dodecyl benzene sulfonate (SDBS) and sodium oleate (OAS) were studied by experimental and computer simulation methods. With the spinning drop method and the suspension drop method, the interfacial tension of oil/aqueous surfactant systems was measured, and the influence of surfactant concentration and salinity on the interfacial tension was investigated. The dissipative particle dynamics (DPD) method was used to simulate the adsorption of SDBS and OAS at the oil/water interface. It was shown that it is beneficial to decrease interfacial tension if the hydrophobic chains of the surfactant and the oil have similar structure. The accession of inorganic salts causes surfactant molecules to form more compact and ordered arrangements and helps to decrease the interfacial tension. There is an osculation relation between interfacial density and interfacial activity. The interfacial density calculated by molecular simulation is an effective parameter to exhibit the interfacial activity.